Adhesives have been used for a variety of marking, holding, protecting, sealing and masking purposes. Adhesive tapes generally comprise a backing, or substrate, and an adhesive. One type of adhesive which is particularly preferred for many applications is represented by pressure sensitive adhesives.
Pressure-sensitive tapes are virtually ubiquitous in the home and workplace. In its simplest configuration, a pressure-sensitive tape comprises an adhesive and a backing, and the overall construction is tacky at the use temperature and adheres to a variety of substrates using only moderate pressure to form the bond. In this fashion, pressure-sensitive tapes constitute a complete, self-contained bonding system.
Pressure sensitive adhesives (PSAs) are well known to one of ordinary skill in the art, and according to the Pressure-Sensitive Tape Council, PSAs are known to possess properties including the following: (1) aggressive and permanent tack, (2) adherence with no more than finger pressure, (3) sufficient ability to hold onto an adherend, and (4) sufficient cohesive strength to be removed cleanly from the adherend. Materials that have been found to function well as PSAs include polymers designed and formulated to exhibit the requisite viscoelastic properties resulting in a desired balance of tack, peel adhesion, and shear holding power. PSAs are characterized by being normally tacky at room temperature (e.g., 20° C.). PSAs do not embrace compositions merely because they are sticky or adhere to a surface.
These requirements are assessed generally by means of tests which are designed to individually measure tack, adhesion (peel strength), and cohesion (shear holding power), as noted in A. V. Pocius in Adhesion and Adhesives Technology: An Introduction, 2nd Ed., Hanser Gardner Publication, Cincinnati, Ohio, 2002. These measurements taken together constitute the balance of properties often used to characterize a PSA.
With broadened use of pressure-sensitive adhesive tapes over the years, performance requirements have become more demanding. Shear holding capability, for example, which originally was intended for applications supporting modest loads at room temperature, has now increased substantially for many applications in terms of operating temperature and load. Indeed, many applications require pressure sensitive adhesives to support a load at elevated temperatures, typically in the range of from 70° C. to 90° C., for which high cohesive strengths are required. So-called high performance pressure-sensitive tapes are those capable of supporting loads at elevated temperatures for 10,000 minutes. Increased shear holding capability may generally be accomplished by crosslinking the PSA, although considerable care must be exercised so that high levels of tack and adhesion are retained in order to retain the aforementioned balance of properties.
In addition, when used as attachment devices for a variety of assembly and manufacturing applications, such as interior or exterior automotive mounting of panels and molding, or in the construction industry, pressure sensitive adhesives are additionally required to provide good adhesion performance to uneven or irregular surfaces. In that context, the application of PSA foam tapes to uneven or irregular surfaces is a recognized challenge, since the conformability of the adhesive tape is limited. In some specialized applications, such as taped seal on body and weather-strip tape applications for the automotive industry, successful tape application is rather challenging, especially in small radii in car bodies and critical topologies such as spot welds or other surface structures, since the adhered tape needs to combine two rather contradicting requirements. On the one hand, the tape has to resist high deformation forces, therefore high stress relaxation capabilities and good cohesive strength are required. On the other hand, the same unique tape needs to provide sufficient conformability to the uneven or irregular surface, and therefore excellent surface wetting capabilities are required for the tape.
It is therefore a recognized and continuous challenge in the adhesive tapes industry to provide pressure sensitive adhesive assemblies combining good adhesion, good stress relaxation and good cohesion properties. In order to optimize the adhesion of the PSA assembly to uneven or irregular substrates, an excellent surface wetting is additionally required.
Partial solutions have been described in the art, whereby a non- or very low crosslinked adhesive is applied to a surface and then post-cured, so that, after an adequate surface wetting, the cohesive strength can be built up. In that context, the so-called “semi-structural tapes” described e.g. in U.S. Pat. No. 5,721,289 (Karim et al.) have been used. These systems are based on post-curable epoxy functionalities and specifically require using a superacid which is activated by UV irradiation as triggering energy. Other known post-curable systems are based on the so-called “DICY-chemistry” described e.g. in EP-A1-0798354, wherein an epoxy-amine curing reaction is triggered with heat. The post-curable systems described in the art typically require complex handling of pre-adhesive compositions including carefully controlled curing or crosslinking steps. Also, the described partial solutions generally do not provide industrially viable solutions for the production of pressure sensitive adhesives having acceptable characteristics, in particular for specialized applications, such as taped seal on body and weather-strip tape applications, whereby successful tape application is required on challenging topologies such as spot welds.
Without contesting the technical advantages associated with the pressure sensitive adhesive films known in the art, there is still a need for a cost-effective pressure sensitive adhesive assembly having versatile adhesion and cohesive characteristics, in particular with respect to adhesion, stress relaxation and cohesion properties, whilst providing excellent surface wetting properties on substrates provided with uneven or irregular surfaces.
Other advantages of the pressure sensitive adhesive (PSA) assembly and methods of the disclosure will be apparent from the following description.